1. Field of the Invention
The present invention relates to technology of measuring terahertz (THz) wave in time domain.
2. Description of the Related Art
THz wave is electromagnetic wave or radiation including a frequency component in a frequency range between 0.03 THz and 30 THz. THz-TDS (THz-Time Domain Spectroscopy) apparatus can acquire the intensity of instantaneous electric field of THz wave ultra-short pulse reaching a detector. This apparatus records a change in the instantaneous electric field intensity caused by changing time at which the ultra-short pulse reaches the detector, and acquires the time-domain waveform of THz wave. Since a signal intensity corresponding to the electric field intensity is very weak, the signal is acquired by using a small signal measuring device or detector such as a lock-in amplifier. Where signal detection is executed by the lock-in amplifier in the THz-TDS apparatus, THz wave is modulated at frequency fs so that the lock-in amplifier can detect a component at the frequency fs.
The signal thus detected is treated as the instantaneous electric field intensity of THz wave. At this time, the lock-in amplifier multiplies the signal of modulated electric field intensity with a reference signal at the frequency fs. The lock-in amplifier picks out a difference frequency component by a filter from derived components at sum frequency (2fs) and difference frequency (DC).
Regarding the signal measured by such a small signal measuring device, noise due to intensity variation of the ultra-short pulse and signals generated by the measuring system (i.e., thermal noise, noise specific to the system and the like) are superposed on the signal relevant to the above-described electric field intensity. Accordingly, where the time-domain waveform of THz wave is to be formed, each measurement data is liable to fluctuate about signals to be acquired, as illustrate in FIG. 7. Such signal fluctuation is a reason for raising a noise floor when the time-domain waveform of THz wave is converted into information in the frequency domain. Rise of the noise floor causes limitation to measurement band, so that suppression of unwanted signal components is desired.
To cope with the above issue, the following method has been proposed. In this method, plural time-domain waveforms of THz wave are summed, and arithmetic mean of the sum is calculated. Japanese Patent Laid-open No. 2006-266908 discloses a method in which arrival times of the ultra-short pulse at the detector are closely set to acquire a single time-domain waveform, and after plural time-domain waveforms are formed from the single time-domain waveform, the plural time-domain waveforms are subjected to arithmetic mean processing.
In the above method of obtaining the arithmetic mean from plural time-domain waveforms of THz wave, measurement time increases as times of measuring THz wave increases. Japanese Patent Laid-open No. 2006-266908 discloses a means for solving this issue. In this method, arrival timing of the ultra-short pulse at the detector is controlled by a mechanical stage. More specifically, a retro-reflective optical system (a delay optical system) is arranged on the mechanical stage to regulate an optical path through which the ultra-short pulse is transmitted. Intervals between measuring points constituting the time-domain waveform of THz wave correspond to the maximum frequency in the frequency domain. For example, where spectrum in a frequency domain of several tens THz is to be obtained, the mechanical stage is controlled with precision of several microns. When the time-domain waveform of THz wave is to be acquired, measurement is performed by moving the mechanical stage with this positioning precision over a range of several millimeters. Measurement time of THz wave mainly depends on this moving velocity of the delay optical system.
In the technology of Japanese Patent Laid-open No. 2006-266908, the mechanical stage is moved with very short intervals between measuring points. In such a case, the mechanical stage is to be moved with very fine positioning precision. Accordingly, where the time-domain waveform of THz wave is to be rapidly acquired, the mechanical stage needs to be moved at high speed over a long distance while maintaining the fine positioning precision. However, this is not easy to achieve.
Further, the lock-in amplifier has to have a time constant capable of responding to a signal change appearing when the mechanical stage is moved at high speed. More specifically, the time constant should be decreased. However, depending on the modulation frequency fs and the velocity of the mechanical stage, the time constant can come close to the sum frequency 2fs when the time constant is decreased. As a result, there occurs a fear that signals near the sum frequency component cannot be sufficiently attenuated, and the noise floor in the frequency domain may rise. For example, in FIG. 6B, the frequency characteristic of the small signal detecting portion depends on the above time constant, and sufficient attenuation of the sum frequency component (2fs) becomes difficult if this frequency characteristic approaches the 2fs component. Consequently, for example, as illustrated in FIG. 5B, fluctuation of the signal supplied from the small signal detecting portion increases, and the noise floor rises. This causes limitation to the measurement band of the frequency spectrum. This is an issue specific to the THz wave measurement, occurring where the time-domain waveform of THz wave is to be acquired at high speed by using the lock-in amplifier for obtaining a DC component. Namely, since the frequency spectrum is to be acquired, both high-speed measurement and signal stability are required.
As described in the foregoing, in the measurement of THz wave, reduction of noise floor and measuring time is desired.